The Camera view shows the current scene as seen from the currently active camera’s view point.
It can be activated by pressing Numpad0.
The Camera view can be used to virtually compose shots and preview how the scene will look when rendered.
The rendered image will contain everything within the dashed line.
In this view you can also set the Render Border which defines the portion of the 3D View to be rendered.

This feature allows you to position and orient the active camera to match your current viewport.

Select a camera and then move around in the 3D View to a desired position and direction for
your camera (so that you are seeing what you want the camera to see). Now press
Ctrl-Alt-Numpad0 and your selected camera positions itself to match the view,
and switches to camera view.

By enabling Lock Camera to View in the View panel of the Properties region,
while in camera view, you can navigate the 3D View as usual,
while remaining in camera view. Controls are exactly the same as when normally moving in 3D.

To perform these camera moves, the camera must first be selected,
so that it becomes the active object (while viewing through it,
you can RMB – click on the solid rectangular edges to select it).
The following actions also assume that you are in camera view Numpad0!
Having done so, you can now manipulate the camera using the same tools
that are used to manipulate any object:

Roll

Press R to enter object rotation mode. The default will be to rotate the camera in its local Z axis
(the axis orthogonal to the camera view), which is the definition of a camera “roll”.

Vertical Pan or Pitch

This is just a rotation along the local X axis. Press R to enter object rotation mode, then X twice
(the first press selects the global axis, pressing the same letter a second time selects the local axis –
this works with any axis;
see the axis locking page).

Horizontal Pan or Yaw

This corresponds to a rotation around the camera’s local Y axis.
Press R, and then Y twice.

Dolly

To dolly the camera, press G then MMB (or Z twice).

Sideways Tracking

Press G and move the mouse (you can use X twice or Y
to get pure-horizontal or pure-vertical sideways tracking).